The reuse of waste products is an important concern of many industries. In the hollow glass industry, the introduction of recovery cullet produced by the collection and grinding of bottles and other glass containers into molten glass charges for recycling represents a particularly desirable goal.
Recycling operations, however, often present the risk of compromising manufacturing quality. In the glass industry, the fact that recovery glass does not have a uniform composition does not limit the proportion of cullet that can be incorporated into the molten glass charges. A major problem posed by the reuse of glass waste products containing a considerable proportion of foreign material, such as stones or infusible pieces of pottery, which is present due to collection processes and successive handling, is the possibility that an unacceptable percentage of foreign material will carry over into the new glass products and create deficiencies in the products of which they will ultimately become a component. To avoid this problem, the standard solution is to sort through the glass waste before reusing it.
A further constraint on the process of purifying recovery glass is an economic one. It is not feasible to recover glass by sorting through household trash from normal collections, and glassmakers can only set up exclusive collection routes in exceptional cases.
In order to secure sufficient tonnage for recovery, it is therefore necessary to arrange for special collection by the usual collection services for storage and, ultimately, routine delivery to the glassworks. Glass collected in this way, due to extensive handling and uncertain cleanliness conditions, contains many broken articles consisting of variable-sized fragments intermixed with light scrap materials such as packing trash, labels, papers, plastic cups, non-ferrous metal elements, lead, tin, and aluminum, or other impurities introduced during the intermediate storage stage, such as pieces or fragments of non-ferrous metals, stones or larger objects of various types. Even when direct collection processes are carried out under optimal conditions of cleanliness, the existence of various packaging elements for the original glass products makes it impossible to completely eliminate the presence of foreign materials in the collected used-products.
Due to the presence of these impurities, the recovered raw glass materials must be purified. The most effective method of purification used to date has been the technique of manual sorting. This process has many drawbacks, including unpredictable results due to operator error and high costs due to the necessity of a series of successive treatments--whose yield of unwanted elements remains fairly constant while absolute effectiveness diminishes with each treatment--to attain a product having a somewhat acceptable level of impurities.